The present invention relates to improvements in one-way vision display panels of the kind constructed from perforated plastic sheet material and which include an image or pattern which is only visible when the display panel is viewed from one direction and wherein the display panel permits substantially unobstructed through-viewing when viewed from the opposite direction. More particularly, the invention relates to a method for transferring a printed image onto a display surface of the perforated membrane material in such a manner whereby the through-viewing capability of the one-way vision display panel is not adversely effected.
One-way vision display panels of the type which are constructed from plastic film material and contain a printed image which is visible when viewed from one direction and which appears transparent when viewed from a second, opposite direction are known from the prior art. Such one-way vision display panels are advantageously used in advertising since they may be easily applied to and displayed on any smooth transparent surface, such as the windows of buildings, buses, streetcars, trucks and the like.
In accordance with conventional one-way vision display panel design, the display image is formed as a pattern of two-color opaque dots which are applied by screen, litho or similar printing process along an interface surface between two adjoining transparent plastic panels. The opaque dots appear white or light in color on one side and black on the other. Light incident on the light color side of the panel is scattered and reflected thereby permitting an image formed by the dot pattern to be seen when viewed from this direction. Light incident on the opposite or black side of the panel is absorbed such that the light transmitted through the transparent portions of panel permit through-viewing in the direction from the black color side to the light color side.
A one-way vision display panel constructed as a perforated plastic panel or membrane having a black rear surface and a white opaque front surface offers superior optical through-vision properties as compared to the conventional one-way vision display panels of the prior art mentioned at the outset. The reason for this is that fewer optical losses due to diffraction and refraction are experienced when light is transmitted virtually unobstructed through the holes of the perforated plastic film material as compared to when light is transmitted through the numerous transparent plastic and adhesive layers of the prior art one-way vision panels.
A problem arises, however, when using conventional printing processes, such as liquid ink silk screen, litho or similar inking processes, for printing an image or pattern on the white opaque front side surface of a perforated plastic panel or membrane. The ink used in any of these conventional inking processes has a tendency to travel or bleed into the outer and upper perimeter of the holes of the perforated plastic membrane thereby making the image printed on the opaque white side visible from the rear or black side. This means that when looking from behind the panel (i.e. when looking into the rear or black side for viewing through the panel) the presence of the ink in the side walls of the holes creates a corona effect, i.e. the ink in the holes gives rise to an undesirable halo or phantom image which is seen when viewing the display panel from behind, i.e. in the through-viewing direction.
Accordingly, there is a definite need in the art for a method of accurately printing an image onto a surface of a one-way vision display panel constructed as a perforated plastic panel or membrane which overcomes the problems of the prior art.